JavaScript decorator pattern

I don’t care how many other places this is on the net, file this one under “so I won’t need to look ever again.”

I needed to augment an existing JavaScript function by applying a decorator to it, where I needed to dynamically add special behavior for this one page.  Ideally, I could have bound to some event, but a decorator worked just as well.  Instead of doing some custom JavaScript for decorating this one method, I modified the Function prototype, to add two new methods, “before” and “after”:

Function.prototype.method = function(name, func) {
    this.prototype[name] = func;
    return this;

Function.method('before', function(beforeFunc) {
    var that = this;
    return function() {
        beforeFunc.apply(null, arguments);
        return that.apply(null, arguments);

Function.method('after', function(afterFunc) {
    var that = this;
    return function() {
        var result = that.apply(null, arguments);
        afterFunc.apply(null, arguments);
        return result;

Each decorator method captures the original method (in the “that” variable) and returns a new method that calls the original and new method passed in, all without modifying the original method.  With these new Function prototype methods, I can do some highly interesting code:

var test = function(name) { alert(name + ' is a punk.'); };

test('Nelson Muntz');

test = test.before(function(name) { alert(name + ' is a great guy.'); });


test = test.after(function(name) { alert(name + ' has a fat head.'); });


We see from this code, in succession, alerts:

  • “’Nelson Muntz is a punk”
  • “Milhouse is a great guy”
  • “Milhouse is a punk”
  • “Jimmy is a great guy”
  • “Jimmy is a punk”
  • “Jimmy has a fat head”

This code was brought to you by lessons learned from the awesome JavaScript book, “JavaScript: The Good Parts”, and the magic of closures.

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